Apple introduced its first foldable iPhone on 9 September 2026. The name is iPhone Duo. Pre-orders start at 5 a.m. PT on 16 October, which is 20:00 the same day in Beijing. The first wave, more than 70 countries and regions including China, goes on sale 23 October. Twenty-eight more regions follow on 30 October. It ships with iOS 27.1. US pricing starts at $1,999 for 256GB. Specs follow the Apple Newsroom release.
The question today is not whether to buy one. It is what happens to the iOS apps already on the App Store: will the foldable make them obsolete, and which screens have to change. Apple’s developer note is direct. Every app runs on Duo. Apps built with the iOS 27 SDK or later resize as the device folds. Close it and the app returns to the outer display. Open it and the app expands onto the inner display. What stretches, clips, or lands in the wrong place are hardcoded screen sizes, fixed orientations, and layouts that assume a device.
Both displays share the same aspect ratio, so content can scale. That is why many lists and forms do not need a new drawing. The inner display is 7.6 inches, 50 percent larger than iPhone 18 Pro Max. The outer display is 5.4 inches, about 90 percent of the screen area of iPhone 18 Pro, and closed it is about the size of a passport. Stretch one column across that inner display and a feed looks empty. A split layout is what the canvas is for.
Our 9 September note used the name Ultra that was circulating that day. The product name in the press release is iPhone Duo. This piece is only about the interface of apps that already exist.
Existing iOS apps will not be obsolete. The business logic does not get rewritten. What needs a new layout is any screen that assumes the screen is always one size: hardcoded frames, phone/pad branches, custom bars, plus camera, video, and workspaces that should split on the inner display.
It runs. That is not a finished layout
Prepare for iPhone Duo puts the floor in the first paragraph. Every app runs. Apps built with the iOS 27 SDK and later expand onto the inner display when the device opens and return to the outer display when it closes. Hardcoded screen sizes, fixed orientations, and device assumptions break the layout.
Calling that “obsolete” overshoots. Accounts, APIs, in-app purchase, and push do not get rewritten because of a hinge. On 23 October the old binary still launches. What the buyer may see is an old phone window in the middle of the inner display, with empty space around it. For a 7.6-inch display that starts at $1,999, that is not done.
The Human Interface Guidelines narrow the job the same way. Supporting poses does not mean a custom layout for each fold angle. A compact-width layout for the outer display and a regular-width layout for the inner display cover the fundamentals. Do not reinvent the app when it resizes. Let the existing layout expand into the space. See Preparing your app for iPhone Duo.
Three SDK tiers decide the inner display
The same product can look different depending on the SDK you built with. Apple’s comparison is this:
| Build SDK | Open (inner display) | Closed (outer display) |
|---|---|---|
| iOS 26 or earlier | Centered window, empty space around it | Content sits left of the status bar and camera |
| iOS 27 | Fills most of the inner display, still avoids the status bar on the right | Returns to the outer display, with less empty space than the older SDK |
| iOS 27.1 or later | Uses the full display. Toolbars and tab bars go vertical under the status bar | The same vertical bars, switching displays as it folds |
Duo ships with 27.1. Staying on an older SDK only so the app “installs” leaves the inner display letterboxed. Before you ship a 27.1 SDK build, confirm that content still works when the bars become vertical. Otherwise full-screen content overlaps the system bars.
Which apps must change the interface
Do not cut this by category first. Search the project for the assumptions below. If the search hits, that screen needs a layout change. If it does not, the screen mostly follows system containers.
Hardcoded screen size
Search for UIScreen.main. A fold changes the app’s size in the middle of a session, and that size often is not the size of the screen.
- Replace
UIScreen.main.boundswithview.boundsorwindow.bounds. In SwiftUI, useGeometryReaderoronGeometryChange. - Reading the size once at launch or in
viewIsAppearingis not enough. Do size-dependent layout inlayoutSubviewsorviewDidLayoutSubviews, and useviewWillTransition(to:with:)when the size changes. - Take scale from
traitCollection.displayScaleor@Environment(\.displayScale). - Replace
UIWindow(frame: UIScreen.main.bounds)withUIWindow(windowScene:). Do not store aUIScreen. - A table of known heights such as
bounds.height == 844will not match Duo.
Orientation and device branches
On Duo, device orientation does not tell you whether the app is wider than it is tall. Remove layout that depends on UIDevice.current.orientation, statusBarOrientation, and interfaceOrientation, including windowScene.effectiveGeometry.interfaceOrientation. Compare the width and height of the space you have.
With Duo and resizable iPhone Mirroring, an iPhone app can have any combination of size classes. On the inner display the app should look like a natural extension of its iPad layout, not a second product.
- Do not choose a layout with
userInterfaceIdiom == .phoneor== .pad. Choose by size class or available space. - Do not treat regular width as “this is an iPad.” That branch now runs on the iPhone inner display. The outer display in portrait is still compact width and regular height, like any iPhone.
- If the iPad branch still contains hardcoded sizes meant only for a large screen, the inner display will run those sizes as they are.
Custom bars go vertical
On 27.1, system tab bars, navigation, and Lock Screen controls move to the side, under the status bar, so vertical space stays with the content. Opened, the Home Screen shows two pages, with Today View — a vertical list of widgets — on the left. The status bar becomes a circle in the corner. The Dynamic Island is redesigned to sit vertically on the side of both displays.
A custom top bar, bottom bar, or floating button that still assumes “one horizontal bar along the bottom” will fight the vertical system bars, or cover the status bar on the right of the inner display. Custom bars and edge-to-edge canvases should use the iOS 27.1 ReservedRegion API in SwiftUI, or UIViewReservedRegion in UIKit, so the fold and camera occlusions stay clear. System split views avoid the fold on their own. A hand-built HStack or ZStack does not.
When the device is partly open, a system arrangement view places the primary view on one side of the fold and the secondary view on the other. Fully open or closed, with no division reserved region, an overlay arrangement stays stacked. A custom stack has to handle that change itself.
Video, camera, reading, calls
Half folded, the inner display behaves like two halves. Media, camera, reading, and video calls should put content on one half and controls on the other, not across the crease.
Apple’s own camera already uses both displays: Duo Preview shows the rear-camera selfie on the outer display, Kid Cue plays an animation there, and Duo FaceTime lets someone nearby join on the outer display. A third-party camera, filter, or live view with a single preview canvas will cut the subject on the crease once the device opens.
Hero images and video set to .scaleAspectFill or .aspectRatio(contentMode: .fill) can lose the subject on the wider inner display. Choose fill or fit from the current size class or aspect ratio, or set a focal point. In the press release, Netflix uses the outer display for clips and the large screen once opened; Zoom splits shared content and participants on the inner display. That is a layout change: the same content, a different container, not a new business.
Split inside, one column outside
iOS 27 puts Split View on iPhone for the first time: two apps side by side, or two windows of the same app, such as two Safari windows. A person can also talk to Siri AI on one side while looking at content on the other, and save a pair of apps to reopen later.
Mail, documents, and a Slack-style workspace waste the inner display if they stay one stretched timeline. Use NavigationSplitView in SwiftUI or UISplitViewController in UIKit: a sidebar or two columns on the inner display, a single column on the outer display, the same path as regular versus compact on any other iPhone. Prefer an even number of grid columns so the fold can divide them. Keep each pane wide enough to read while the device is folding.
The press release names Slack as already bringing the whole workspace onto the inner display. A collaboration tool with no iPad split is the screen most worth scheduling before 23 October. It is still layout, not a rewrite of message sync.
What can wait
| Interface | If you change nothing by 23 October | Rebuild? |
|---|---|---|
| Settings, forms, system lists | They work. Older SDKs letterbox the inner display. A newer SDK stretches one column | No rebuild. Move to the 27.1 SDK and check that vertical bars do not cover text |
| Adaptive layouts that already support iPad | The inner display takes the regular-width path | A small change. Remove “regular width means iPad only” |
Hardcoded 390/844 and UIScreen.main |
Stretch, clipping, controls in the wrong place | The layout must change. The business logic does not |
| Camera, video calls, reading, full-bleed video | The subject can sit on the crease. Fill can crop a face | Change that screen: halves, focal point, safe area |
| Mail, documents, collaboration | They work, as one stretched timeline on the inner display | A split is worth doing. The outer display stays one column |
| Games, maps, design canvases | A new ratio shows edges, or crops the subject and the HUD | Change the canvas and safe area, not the rules of the game |
An internal tool that is happy as a normal phone on the outer display, with letterboxing inside, can keep running. The App Store page is a separate decision. App Store Connect can preview iPhone Duo assets, and a featuring nomination can say the app supports every pose. That is the store page, not a second codebase.
Do not keep three unrelated interfaces for closed, open, and half folded. The guidance is one layout that expands with the space.
How to test before you ship
In Xcode, ask the coding assistant to get my app ready for iPhone Duo. It scans for the patterns above and suggests a fix. If you use another agent, export the skill with xcrun agent skills export. The skill misses some cases. Hardcoded heights and custom bars still need a person.
Three places to test. Apple calls the Duo simulator the full native experience:
- The iPhone Duo simulator in Xcode 27.1
- The resizable iOS simulator in Device Hub
- iPhone Mirroring on macOS 27, with the window dragged to extreme sizes in both directions
Cover at least six poses: closed, open, and half folded, each in portrait and landscape. Then add system Split View. For a custom camera or video screen, look again at whether the crease cuts the subject when the device is half folded.
- Install Xcode 27.1 and the Duo simulator on a separate Mac. Daytime meetings and release builds should not share that notebook.
- Run the skill scan, then look by hand at custom bars, full-bleed video, and the half-folded pose.
- If there is no regular-width layout, do not start the split on the morning pre-orders open, 16 October.
- Schedule store screenshots apart from the code. Missing screenshots do not remove the old binary from Duo.
The monthly bill for a Mac, the Developer Program, and CI is in the 14 September cost piece. Duo does not zero that table. It only means the 27.1 SDK and the simulator need a clean machine.
FAQ
Will an existing iOS app still open on 23 October if it is not updated?
Yes. Apple says every app runs. Apps built with the iOS 26 SDK or earlier appear centered with empty space when open, and sit to the left of the status bar and camera when closed. Opening is not a finished layout.
Do you have to rewrite the app for the foldable?
No. Accounts, APIs, and business logic stay. What changes are layout assumptions: UIScreen.main, fixed orientation, phone/pad branches, hardcoded width and height, and custom toolbars. Apps that already use system containers and resize on iPad have the smallest job.
Which kinds of apps should schedule a UI change first?
Camera, video calls, reading, and full-bleed video need the fold and the half-folded pose. Mail, documents, and collaboration tools should split on the inner display and return to one column on the outer display. Games, maps, and design canvases need a check for stretch and safe areas. Standard lists and settings can ship with the SDK bump.
If the app is iPhone-only, with no iPad version, will it fail to run?
It will still run. The outer display in portrait is compact width. The inner display is regular width. If regular width is gated as iPad-only, those screens appear on the Duo inner display. If there is no regular layout, a newer SDK stretches the single column and an older SDK letterboxes it.
Which system does iPhone Duo ship with?
The press release says iOS 27.1. Filling the inner display, with toolbars and tab bars vertical under the status bar, is the iOS 27.1 SDK and later. The iOS 27 SDK still avoids the status bar on the right edge.
When do pre-orders and sales start?
Pre-orders start at 5 a.m. PT on 16 October, 20:00 the same day in Beijing. Availability starts 23 October in more than 70 countries and regions, including China, and 30 October in 28 more. US pricing starts at $1,999 for 256GB.
Closing
The foldable does not retire existing iOS apps. It retires the assumption that the screen is always one size. Before 23 October, the screens worth changing are hardcoded sizes, device branches, custom bars, plus camera, video, and a split on the inner display. Lists and settings can follow the 27.1 SDK. Test in the Xcode 27.1 Duo simulator, on a Mac that is not the daily machine.
Put Xcode 27.1 and the Duo simulator on a separate Mac
Closed, open, and half folded each need a pass. That should not share the notebook that has meetings on it all day.